Radiographic images are produced by the detection of X-rays or gamma rays that pass through the object (e.g. the cargo in a truck or container) being inspected. The density, atomic number and the total amount of material that is present determine how much of the radiation is attenuated and, therefore, the nature and type of radiographic image produced. Thus, in addition to determining the average absorption of the X-ray or gamma-ray photons as they travel along the various X-ray paths, it is possible to derive information about the characteristics of the material. The identification of areas in the image where high-Z materials are present is of specific security interest related to the detection of certain classes of weapons of mass destruction (WMD). Radiographic images produced by conventional X-ray and gamma-ray screening systems are typically incapable of determining whether a region contains high-Z material(s). Instead, an inspector examines the image to determine if there are any areas considered suspicious due to their shape, symmetry, size, attenuation or transmittance, etc. Cargo containing suspicious areas must have the cargo contents removed for manual examination. Inspectors must make their decisions by balancing the competing objectives of trying to make certain that all actual threats are detected while maintaining a low false alarm rate to limit the amount of cargo requiring physical inspection so that the stream of commerce is not unduly impacted.
The evaluation of radiographic images by an inspector is subject to human factors that can affect threat detection. For example, threat detection has been found to vary for different inspectors due to such issues as experience, differences in innate perceptive capabilities, eye/mind fatigue from examining a large number of images, and other such hindrances.
Also, the time required to analyze a given image depends on the number of areas or objects initially deemed as being suspicious by the screening system. A typical image searching/threat detection procedure for an inspector consists of quickly reviewing the image for highly attenuating objects by looking for either high levels of attenuation or low levels of transmission. For example, any given image may contain one or more highly attenuating objects or areas that need to be examined in detail. For each object or area, the inspector manually creates contrast enhancements using an input device, such as a mouse. Each object then has to be evaluated for its total attenuation (or transmission) value whereby the inspector selects a region of interest within the object or area and estimates the average pixel value which reflects the total attenuation (or transmission) of the X rays or gamma rays along that path. Before the net attenuation (or transmission) of the object can be estimated, the attenuation (or transmission) of the surrounding background materials has to be analyzed. Then, to generate an estimated net attenuation (or transmission) of the object, the background must be subtracted from the total attenuation. Finally, the inspector must examine the shape and size of the object, and combine these estimates with the estimated net attenuation (or transmission) to reach a conclusion of whether the object represents a threat. This procedure would typically have to be repeated for each suspicious object or area within an image. If done accurately, this is a very time-intensive procedure.
For example, U.S. Pat. No. 7,366,282, assigned to Rapiscan Systems, Inc. and incorporated herein by reference, is “directed towards a method for identifying an object concealed within a container, comprising the steps of generating a first set of data using a first stage X-ray inspection system; processing said first set of data using a plurality of processors in data communication with the first stage inspection system; identifying at least one target region from said processed first set of data; positioning an inspection region relative to the target region wherein the inspection region at least partially physically coincides with the target region; generating the inspection region through a second stage inspection system; and producing a second set of data having a X-ray signature characteristic and fluorescence signature characteristic of the material in the inspection region.” Further, “[i]n another embodiment, the present invention comprises a single stage inspection system comprising an X-ray diffraction and fluorescence system. Contraband, high-Z or other illegal material located within a target object is identified using a radiation source by passing a target object into a C-shaped inspection system; directing an X-ray beam from said radiation source toward a target object; detecting a diffraction signal using a diffraction detector head; detecting a fluorescence signal using a fluorescence detector head; and identifying contraband material using said diffraction signal and said fluorescence signal. The method can further comprise the steps of: generating an image of said target object; analyzing the image using an algorithm to evaluate regions of objects based upon a threshold level; segmenting said image into regions based upon criteria; further inspecting selected regions satisfying certain criteria to determine their size and shape; comparing said selected regions to threat criteria; and issuing an alarm to an inspector when an object is determined as matching said threat criteria in said comparing step.”
In another example, U.S. Pat. No. 6,347,132, assigned to AnnisTech, Inc. and incorporated herein by reference, discloses “an executable routine 50 for automatically detecting nuclear weapons materials. This routine is preferably executed by the signal processor and controller 28 (FIG. 1). Step 52 is performed to sample each of the individual detector elements of the transmission detector 22 (FIG. 1) as the object under inspection 12 (FIG. 1) is scanned relative to the fan beam 20 (FIG. 1), and digitize and store the sampled values. Test 54 performs a threshold detection on the sampled values to identify any areas of unusually high absorption within the image of the object under inspection. That is, since the nuclear weapons materials absorb x-rays significantly more than any other materials, the magnitude of the sampled signals associated with areas within the object under inspection having nuclear weapons materials will be significantly different than the surrounding areas. Therefore, threshold detection is a suitable automatic detection technique. Alternatively, spatial frequency analysis may also be used to detect large changes in the sampled signal magnitude, which may then be analyzed to determine whether or not the large changes in magnitude are consistent with nuclear weapons materials. In any event, detection of the nuclear weapons materials is automatic. Similarly, the region of high attenuation identified in the transmission image is examined in the scatter image (if the pencil beam system is employed). A negative result in the scatter image reinforces the result from the transmission beam analysis. If nuclear weapons' materials are detected, step 56 provides a warning annunciator that may be displayed on the display, initiates an audio alarm, or provides other suitable warning devices.”
In yet another example, U.S. Pat. No. 7,492,682, assigned to GE Homeland Protection, Inc. describes “[a] method for inspecting a container for contraband, said method comprising: positioning the container on a platform configured to support the container, the platform rotatably coupled to a frame that is movably coupled to a base defining an axis, the frame movable with respect to the base in a direction parallel to the axis, and the platform movable with the frame and rotatable with respect to the frame about the axis; producing X-ray beams having at least one energy distribution and transmitting the X-ray beams through the container as the container rotates about the axis and moves in a direction parallel to the axis; detecting the X-ray beams transmitted through the container with an array of detectors to generate signals representative of the detected radiation; and processing the signals to produce images of the container and contents of the container to generate a map for the container including at least one of a CT number, a density and an atomic number corresponding to the contents within the container.”
Conventional prior art threat detection uses various techniques such as conventional radiography, dual-energy imaging, resonant absorption/fluorescence, computed tomography (CT) systems, dual-stage X-ray diffraction and fluorescence systems, to produce radiographic images that are either inspected manually for threat detection and/or analyzed using software routines.
For example, high-energy dual-energy techniques have been employed in conventional systems. Multi-energy inspection employs scanning large objects with two or more energies in the megavoltage region, i.e. 6 MV and 9 MV. This technique is based on the difference of the X-ray attenuation for materials with different atomic numbers. Collecting transmission information for multiple energies enables determining the atomic number of a material along the X-ray path length.
U.S. Pat. No. 7,483,511, assigned to GE Homeland Protection, Inc. describes “[a] method of determining a presence of items of interest within a cargo container, the method comprising: obtaining information from an initial radiation scan of at least one of the cargo container and contents therein, the obtaining comprising: transmitting a screening radiation beam along a screening portion of the cargo container at a screening scan rate; detecting radiation received in response to the transmitting the screening radiation beam; and analyzing the detected radiation received in response to the transmitting the screening radiation beam to develop information regarding the initial radiation scan; identifying a target portion of the cargo container in response to the information obtained, wherein the screening portion is larger than the target portion; transmitting a target radiation beam along the target portion of the cargo container at a target scan rate, the target scan rate being different than the screening scan rate; detecting radiation received in response to the transmitting; analyzing the detected radiation for a presence of items of interest; and in response to the analyzing, generating a first signal indicative of the presence of the items of interest, or generating a second signal indicative of an absence of the items of interest.”
Further, U.S. Pat. No. 7,286,638, assigned to Passport Systems, Inc. describes “[a] method for analyzing material in a voxel of a target, the method comprising: illuminating the voxel with a photon beam; measuring a first number of photons scattered from the voxel in a first energy range and in a first measurement direction; measuring a second number of photons scattered from the voxel in a second energy range and in a second measurement direction; determining a ratio of the first number of photons to the second number of photons; determining an average atomic number of the material in the voxel using the ratio; and generating a signal based upon the average atomic number determined.”
And still further, United States Patent Publication Number 20090323889 describes a “[s]ystem and method for XRD-based false alarm resolution in computed tomography (“CT”) threat detection systems. Following a scan of an object with a megavoltage CT-based threat detection system, a suspicious area in the object is identified. The three dimensional position of the suspicious area is used to determine a ray path for the XRD-based threat detection system that provides minimal X-ray attenuation. The object is then positioned for XRD scanning of the suspicious area along this determined ray path. The XRD-based threat detection system is configured to detect high density metals (“HDMs) as well as shielded Special Nuclear Materials (“SNMs”) based on cubic or non-cubic diffraction profiles.”
The prior art, however, suffers from severe limitations for the high-throughput inspection of large dense cargo. For example, inspection methods based on dual-energy and fluorescent methods have difficulties with threat detection within dense, highly attenuating cargo; CT systems are not practical for inspection large cargo because of size and speed constraints. Further, software routines based on threshold detection have not proven effective due to the inability to distinguish between the presence of high-Z materials and areas that have high attenuation due to their thickness and density.
What is therefore needed is a method for automatically and rapidly analyzing radiographic images, specifically for high-atomic-number (high-Z) materials, where “high-Z” refers to materials in the periodic table of atomic number 72 (Hafnium) and above.
What is also needed is a method for accurately detecting high-Z materials in very large and dense objects (e.g. containers containing metals and other dense cargo) with an inspection and analysis speed that results in minimal additional delay in the clearing of cargo.
What is also needed is a method that implements complementary modules that analyze the radiographic image using both threshold and gradient detection techniques along with characteristic geometric and physical considerations to reduce false alarms while automatically and rapidly rendering “High-Z”/“Clear” decisions.